Method for cleaning a multilayer substrate and method for bonding substrates and method for producing a bonded wafer

ABSTRACT

Disclosed is a method for cleaning a multilayer substrate at least having a silicon single crystal wafer with a SiGe layer epitaxially grown on a surface of the silicon single crystal wafer, where the SiGe layer is an outermost surface of the SiGe layer and then cleaning the multilayer substrate with a first cleaning liquid capable of etching the protective film so that the protective film remains. The protective film prevents roughening of the surface of the SiGe layer while the cleaning is performed. The cleaning is performed. The cleaning is performed so that a thickness of the remaining protective film is from 1 nm to 100 nm.

TECHNICAL FIELD

The present invention relates to a method for cleaning a multilayersubstrate having a SiGe layer, a method for bonding substrates, and amethod for producing a bonded wafer having a Si epitaxial layer on aSiGe layer.

BACKGROUND TECHNOLOGY

In recent years, for satisfying demands for high-speed semiconductordevices, semiconductor devices such as high-speed MOSFET(Metal-Oxide-Semiconductor Field Effect Transistor) in which a Si layerobtained by performing epitaxial growth through a SiGe (SiliconGermanium) layer on a Si (Silicon) substrate is used for a channelregion have been suggested.

In this case, because a SiGe crystal is larger in lattice constant thana Si crystal, tensile strain is generated in a Si layerepitaxially-grown on a SiGe layer (hereinafter, a Si layer in whichstrain is generated in this manner is referred to as a strained Silayer). Energy band structure of a Si crystal is changed by the strainedstress, and as a result, degeneracy of the energy band is removed andhigh-energy band with high carrier mobility is formed. Therefore, theMOSFET that the strained Si layer is used as a channel region showshigh-speed operation property that is about 1.3-8 times higher thanusual.

Because magnitude of the tensile strain generated in the strained Silayer becomes larger along with Ge concentration of the SiGe layer beinghigher, the Ge concentration of the SiGe layer is an importantparameter. Hereinafter, a SiGe layer having Ge composition rate X(0<X<1) is occasionally described as a Si_(1-X)Ge_(X) layer.

As a method for forming such a strained Si layer, besides the abovemethod that an epitaxial method is main, there is a known method forforming a Si_(1-X)Ge_(X) layer on a silicon substrate to be a bondwafer, producing a bonded SOI substrate that a surface of the formedSi_(1-X)Ge_(X) layer of the bond wafer is bonded to a silicon substrateto be a base wafer through an oxide film and thereby to be SOI (SiliconOn Insulator) structure, and then thinning the silicon substrate of thebond wafer to be a strained Si layer, for example, as disclosed inJapanese Patent Application Laid-open (kokai) No. 2001-217430 and No.2002-164520. In this case, as disclosed in Japanese Patent ApplicationLaid-open (kokai) No. 2002-164520, it is also possible that a surface ofthe Si_(1-X)Ge_(X) layer is subjected to thermal oxidation according toneed thereby to be a condensation SiGe layer that the Ge concentrationis enhanced.

In this case, the thinning of a silicon substrate of the bond wafer isperformed with a grinding and polishing method, vapor etching such asPACE (Plasma Assisted Chemical Etching) method, an ion implantationdelamination method (which is also referred to as a smart cut [aregistered trademark] method), or the like.

The ion implantation delamination method is a technology of, implantingat least one of a hydrogen ion and rare gas ions from a surface of abond wafer, namely, a surface of the Si_(1-X)Ge_(X) layer, to form amicro bubble layer inside the bond wafer such as near the surfacethereof, superposing closely the bond wafer in the ion implanted surfaceside on a base wafer through an oxide film, then performing heattreatment (delaminating heat treatment) to delaminate the bond wafer asa thin film so that the micro bubble layer is a cleavage plane (adelaminating plane), and further performing heat treatment (bonding heattreatment) to bond the two wafers tightly to provide a bonded wafer.

In the disclosure of Japanese Patent Application Laid-open (kokai) No.2002-305293, it is disclosed that a separating layer is formed by ionimplantation into a silicon substrate of a bond wafer in which aSi_(1-X)Ge_(X) layer, a silicon layer, and an insulator layer are formedon the silicon substrate. And a surface of the insulator layer of thebond wafer is bonded to a base wafer. Then, a silicon layer of thedelaminated layer that is delaminated at the separating layer andtransferred to the base wafer side is made to be a strained Si layer.

In general, in a bonded substrate such as a bonded wafer, it is desiredthat bonding force of the bonded plane is strong enough to preventgeneration of problems such as delamination at the bonded plane. Ingeneral, evaluation of the bonding force at the bonded plane of thebonded substrate can be performed by an evaluation of surface energy ofthe bonded plane which is proportional to the bonding force. Measurementof the surface energy can be performed by using a razor-blade insertionmethod as disclosed in Japanese Patent Application Laid-open (kokai) No.7-29782.

In the case that a surface of a SiGe layer is bonded to another siliconsubstrate through an oxide film as disclosed in Japanese PatentApplication Laid-open (kokai) No. 2001-217430, particles or contaminantson the surfaces need to be removed by cleaning the bonded plane beforebonding is performed. In the cleaning step, so-called SC-1 cleaning inwhich a mixed aqueous solution of NH₄OH and H₂O₂ (SC-1: StandardCleaning 1) that is one of general cleaning liquids for a siliconsubstrate is used as a cleaning liquid is generally performed.

DISCLOSURE OF THE INVENTION

As described above, in the case that a bonded wafer is produced by usingan ion implantation delamination method, there has been a problem thatcontamination such as organic matter and metal impurities or rougheningof a surface is caused on a surface such as a Si_(1-X)Ge_(X) layer, a Silayer, and an insulator layer which are implanted surfaces when ions areimplanted. And, in the case that a surface of the Si_(1-X)Ge_(X) layeror the like and a base wafer are closely superposed through an oxidefilm after the ion implantation, bonding defects such as voids orblisters are generated in a bonded plane after delaminating heattreatment. Such voids or blisters degrade process yield of producingbonded wafers.

In this case, it is thought that by performing such SC-1 cleaning asmentioned above, organic matter, metal impurities, and the like on thesurface are removed.

By subjecting a silicon substrate to SC-1 cleaning, cleaning effect canbe enhanced because a surface of the silicon substrate is slightlyetched and removed. However, it has become clear that by subjecting asurface of a SiGe layer to SC-1 cleaning, surface roughness of the SiGelayer after being cleaned is larger than that in the case of subjectinga surface of a silicon substrate to SC-1 cleaning. According toinvestigation of the present inventors, it has been found that this iscaused because Ge etching rate is larger than Si etching rate, and thesurface roughness becomes larger along with Ge concentration beinghigher. Therefore, in the case that a bonded substrate is produced bybonding a surface of a SiGe layer and a silicon substrate or the likeafter cleaning, bonding force of the bonded plane lowers. Such loweringof bonding force of the bonded plane causes delamination at the bondedplane in a later step such as thinning of the bond wafer, and loweringof process yield of producing bonded substrates is provoked.

For example, in the above-described production of a bonded wafer, therehas been a problem that when a surface of a Si_(1-X)Ge_(X) layer or thelike after being cleaned and a base wafer are closely superposed throughan oxide film, bonding defects such as voids or blisters are caused atthe bonded plane after the delaminating heat treatment.

Moreover, in the case that a Si_(1-X)Ge_(X) layer is bonded to a basewafer through a Si layer and an insulator layer thereon, there ispossibility that dislocations are caused because lattice relaxationwithin a condensation SiGe layer is not performed sufficiently when Geconcentration is enhanced in the Si_(1-X)Ge_(X) layer. In this case, astrained Si layer directly thereon is also bad in crystallinity.

The present invention has been accomplished in view of the aboveproblems. The first object of the present invention is to provide acleaning method and a bonding method that roughening of a surface of aSiGe layer caused when a multilayer substrate at least having a SiGelayer as an outermost surface layer is cleaned can be prevented, andlowering of bonding force of the bonded plane in the subsequentproduction of a bonded substrate can be prevented.

Moreover, the second object of the present invention is to provide amethod for producing a bonded wafer for preventing generation of bondingdefects such as voids or blisters at the bonded plane due tocontamination such as organic matter or metal impurities adhering to asurface of a Si_(1-X)Ge_(X) layer and roughening of the surface whichare along with an ion implantation when an ion implantation delaminationmethod is used, and preventing generation of dislocations on theSi_(1-X)Ge_(X) layer to grow a strained Si layer of good qualitythereon.

In order to accomplish the above first object, the present inventionprovides a method for cleaning a multilayer substrate at least having aSiGe layer as an outermost surface layer, at least, comprising steps offorming a protective film on a surface of the SiGe layer, and thencleaning the multilayer substrate with a first cleaning liquid capableof etching the protective film so that the protective film remains.

If a protective film is formed on a surface of the SiGe layer and thenthe protective film is cleaned with a first cleaning liquid so that theprotective film remains as described above, roughening of the surfacecan be prevented because the SiGe layer is protected when the cleaningis performed. In addition, because the protective film is slightlyetched and removed with the first cleaning liquid, the cleaning effectcan be high and bonding force of the bonded plane can be prevented fromlowering.

In this case, it is preferable that composition or temperature of thefirst cleaning liquid or the cleaning time is adjusted and therebythickness of the remaining protective film is adjusted.

By adjusting composition or temperature of the first cleaning liquid orthe cleaning time as describe above, thickness of the remainingprotective film can be easily adjusted. Therefore, the thickness of theprotective film can be an appropriate thickness.

Moreover, it is preferable that the cleaning is performed so that athickness of the remaining protective film is from 1 nm to 100 nm.

If a thickness of the remaining protective film is from 1 nm to 100 nmas described above, the thickness is a sufficient thickness forprotecting the SiGe layer from roughening of the surface due to thecleaning. And also in the case that a bonded SOI wafer having a strainedSi layer is produced by bonding thereafter, a thickness from thestrained Si layer to an oxide film can be sufficiently thin.

In these cases, the protective film consisting of Si can be used, andthe first cleaning liquid consisting of a mixed aqueous solution ofNH₄OH and H₂O₂ can be used.

If the protective film consisting of Si is used and the first cleaningliquid consisting of a mixed aqueous solution of NH₄OH and H₂O₂,so-called a SC-1 cleaning liquid, is used, roughening of a surface ofthe protective film to be a bonded plane is prevented and at the sametime cleaning effect can be sufficiently high. Therefore, bonding forceof the bonded plane can sufficiently prevented from lowering.

Moreover, the multilayer substrate cleaned with the first cleaningliquid may be cleaned with a second cleaning liquid which is capable ofetching the protective film and which has a smaller etching rate for theprotective film than the first cleaning liquid so that the protectivefilm is removed and that the SiGe layer is exposed.

If the multilayer substrate cleaned with the first cleaning liquid iscleaned with a second cleaning liquid which is capable of etching theprotective film and which has a smaller etching rate for the protectivefilm than the first cleaning liquid so that the protective film isremoved and that the SiGe layer is exposed as described above, the SiGelayer can be etched with the second cleaning liquid having a smalleretching rate so that roughening of the surface is not caused. Therefore,roughening of a surface of the SiGe layer to be a bonded plane can beprevented and at the same time the cleaning effect can be sufficientlyhigh with the first cleaning liquid, thereby bonding force of the bondedplane can be prevented from lowering.

Moreover, the second cleaning liquid consisting of a mixed aqueoussolution of NH₄OH and H₂O₂ can be used.

If a SC-1 cleaning liquid consisting of a mixed aqueous solution ofNH₄OH and H₂O₂ which has a slower etching rate than the first cleaningliquid is used as the second cleaning liquid as described above,roughening of a surface of the SiGe layer to be a bonded plane isprevented and at the same time the cleaning effect can be sufficientlyhigh, thereby bonding force of the bonded plane can be sufficientlyprevented from lowering.

In these cases, it is preferable that a temperature of the secondcleaning liquid is lower than a temperature of the first cleaningliquid.

If a temperature of the second cleaning liquid is lower than atemperature of the first cleaning liquid, an etching rate of the secondcleaning liquid can be easily small so that roughening of the surface ofthe SiGe layer is not caused. Therefore, roughening of a surface of theSiGe layer to be a bonded plane can be prevented and at the same timethe cleaning effect can be sufficiently high, thereby bonding force ofthe bonded plane can be prevented from lowering.

Moreover, the present invention provides a method for bondingsubstrates, wherein a surface of the SiGe layer or the protective filmwhich is an outermost surface layer of the multilayer substrate cleanedby using the method for cleaning a multilayer substrate as describedabove and a surface of another substrate are bonded directly or throughan insulator film.

If a surface of the SiGe layer or the protective film which is anoutermost surface layer of the multilayer substrate cleaned by using thecleaning method as described above, in which roughening of the surfaceis prevented, which is cleaned sufficiently, and a surface of anothersubstrate are bonded directly or through an insulator film, lowering ofbonding force due to roughening of the surface of the bonded plane canbe prevented. Therefore, troubles such as delamination at the bondedplane do not happen in subsequent steps. Thereby, improvement of processyield of producing a bonded substrate is achieved.

Moreover, for accomplishing the second object as described above, thepresent invention provides a method for producing a bonded wafer, atleast, comprising steps of, forming a Si_(1-X)Ge_(X) layer (0<X<1) on asurface of a silicon single crystal wafer to be a bond wafer, forming aprotective layer on a surface of the Si_(1-X)Ge_(X) layer, implanting atleast one kind of a hydrogen ion and rare gas ions through theprotective layer thereby to form an ion implanted layer, cleaning thebond wafer formed with the ion implanted layer, superposing closely asurface of the protective layer of the bond wafer after being cleanedand a base wafer through an insulator film or directly, then performingdelamination at the ion implanted layer, subjecting a surface of thedelaminated layer transferred to the base wafer side by the delaminationto thermal oxidation thereby to form a thermal oxide film, removing theformed thermal oxide film thereby to expose a condensation SiGe layer inwhich Ge is condensed, and performing epitaxial growth of a siliconsingle crystal layer on a surface of the exposed condensation SiGelayer.

If a Si_(1-X)Ge_(X) layer (0<X<1) and a protective layer are formed inorder on a surface of a silicon single crystal wafer to be a bond wafer,and at least one kind of a hydrogen ion and rare gas ions is implantedthrough the protective layer thereby to form an ion implanted layer andthen the bond wafer formed with the ion implanted layer is cleaned asdescribed above, roughening of the surface of the Si_(1-X)Ge_(X) layerby cleaning can be prevented with the protective layer and at the sametime organic matter or metal impurities adhering to the implantedsurface when ions are implanted can be removed. Therefore, generation ofvoids or blisters at the bonded plane after delaminating heat treatmentcan be prevented. Moreover, if a surface of the protective layer of thebond wafer after being cleaned and a base wafer are closely superposedthrough an insulator film such as a silicon oxide film or directly, itbecomes easy to cause slipping on the interface between the protectivelayer and the base wafer, and in a condensation SiGe layer formed by Gebeing condensed at the Si_(1-X)Ge_(X) layer when the surface of thedelaminated layer is subjected to thermal oxidation thereby to form athermal oxide film, generation of dislocations is suppressed and at thesame time lattice relaxation is performed sufficiently. Therefore,epitaxial growth of a strained Si layer of good quality can be performedon a surface thereof.

In addition, the above-described method for cleaning a multilayersubstrate can be used in the cleaning. In this case, at least, stepsthat a Si_(1-X)Ge_(X) layer is formed on a surface of a silicon singlecrystal bond wafer, a protective layer is formed on a surface of theSi_(1-X)Ge_(X) layer, hydrogen ions and such are implanted through theprotective layer thereby to form an ion implanted layer, and the bondwafer formed with the ion implanted layer is cleaned with a cleaningliquid capable of etching the protective layer so that the protectivelayer remains are performed.

In this case, it is preferable that the X is less than 0.2, and morepreferably 0.15 or less.

If the Ge concentration is less than 20% and particularly 15% or less,the Si_(1-X)Ge_(X) layer with sufficiently small number of dislocationscan be obtained.

Moreover, it is preferable that as the protective layer, at least onekind of a silicon single crystal layer, an amorphous silicon layer, apolysilicon layer, and a silicon oxide film layer is formed.

If the protective layer is at least one kind of these layers, it cansufficiently function as a protective layer and can be easily formed bya vapor growth method or the like. Moreover, if it is a silicon oxidefilm layer, it can be formed by thermal oxidation and can be used as aBOX (Buried OXide) layer after bonded.

And, the ion implantation may be performed from the directionperpendicular to a surface of the protective layer.

Although it is more preferable to perform ion implantation from anoblique direction for preventing channeling which is caused in the ionimplantation, in-plane uniformity of implantation depth distribution isdegraded. If a protective layer is formed and ion implantation isperformed from the direction perpendicular to a surface of theprotective layer according to the present invention, the in-planeuniformity of implantation depth of ions can be enhanced and an ionimplanted layer of good quality can be formed. In particular, if theprotective layer is a silicon oxide film layer, an amorphous siliconlayer, a polysilicon layer, or the like, occurrence of channeling can beeffectively prevented.

In these cases, it is preferable that the insulator film through which asurface of the protective layer of the bond wafer after being cleanedand the base wafer are closely superposed is formed only on a surface ofthe base wafer.

If an interface between the protective layer and the insulator filmformed only on a surface of the base wafer is the bonded plane, slippingis easily caused at the bonded plane. Therefore, lattice relaxation ofthe condensation SiGe layer in which Ge concentration is enhanced byformation of thermal oxide film at the later step is easily performed,thereby generation of dislocations can be suppressed in the condensationSiGe layer.

In these cases, it is preferable that as the base wafer, a siliconsingle crystal wafer or an insulator wafer is used.

If the base wafer is a silicon single crystal wafer as mentioned above,an insulator film can be easily formed by thermal oxidation, a vaporgrowth method, or the like, the base wafer can be superposed closely ona surface of the protective layer of the bond wafer through theinsulator film. Moreover, the protective layer of the bond wafer may bedirectly bonded to an insulator base wafer such as quartz, siliconcarbide, alumina, or diamond according to usage.

In these cases, a temperature when Ge in the SiGe layer is condensed bysubjecting the surface of the delaminated layer to thermal oxidation canbe 900° C. or more.

If a thermal oxidation temperature at the surface of the delaminatedlayer is 900° C. or more as described above, generation of Geprecipitation can be prevented at the interface between the oxide filmand the SiGe layer.

Moreover, it is preferable that a temperature when a Si layer of thesurface of the delaminated layer is made to be a thermal oxide film bysubjecting the surface to thermal oxidation is 1000° C. or less.

If a temperature when a Si layer of the surface of the delaminated layeris made to be a thermal oxide film by subjecting the surface to thermaloxidation is 1000° C. or less, generation of defects such as OSF(Oxidation induced Stacking Fault) can be prevented when damage by theion implantation which remains in the Si layer of a surface of thedelaminated layer is introduced into the thermal oxide film to beformed.

According to the present invention, if a protective film is formed on asurface of a SiGe layer and then the protective film is cleaned with afirst cleaning liquid so that the protective film remains, the SiGelayer is protected during the cleaning and roughening of the surface canbe prevented. In addition, because the protective film is slightlyetched and removed with the first cleaning liquid, cleaning effect canbe high and bonding force of the bonded plane can be prevented fromlowering.

Moreover, if the multilayer substrate cleaned with the first cleaningliquid is cleaned with a second cleaning liquid which is capable ofetching the protective film and which has a smaller etching rate for theprotective film than the first cleaning liquid so that the protectivefilm is removed and that the SiGe layer is exposed, the SiGe layer isetched with the second cleaning liquid having a small etching rate sothat roughening of the surface is not caused. Therefore, roughening of asurface of the SiGe layer to be a bonded plane is prevented and at thesame time the cleaning effect can be very high, thereby bonding force ofthe bonded plane can be prevented from lowering.

Furthermore, if a surface of the SiGe layer or the protective filmcleaned by using the cleaning method as described above and a surface ofanother substrate are bonded directly or through an insulator film,lowering of bonding force due to roughening of the bonded plane can beprevented, troubles such as delamination at the bonded plane do nothappen in subsequent steps, and improvement of process yield ofproducing a bonded substrate is achieved.

Moreover, according to the present invention, if a Si_(1-X)Ge_(X) layer(0<X<1) and a protective layer are formed in order on a surface of asilicon single crystal wafer to be a bond wafer, at least one kind of ahydrogen ion and rare gas ions is implanted through the protective layerthereby to form an ion implanted layer and then the bond wafer formedwith the ion implanted layer is cleaned, roughening of the surface ofthe Si_(1-X)Ge_(X) layer by cleaning can be prevented with theprotective layer and at the same time organic matter or metal impuritiesadhering to the implanted surface when ions are implanted can beremoved. Therefore, voids or blisters can be prevented from beinggenerated at the bonded plane after the delaminating heat treatment.Moreover, if a surface of the protective layer of the bond wafer afterbeing cleaned and a base wafer are closely superposed through aninsulator film or directly, it becomes easy to cause slipping on theinterface between the protective layer and the base wafer, and acondensation SiGe layer formed by Ge being condensed at theSi_(1-X)Ge_(X) layer when the surface of the delaminated layer issubjected to thermal oxidation thereby to form a thermal oxide filmbecomes a layer that generation of dislocations is suppressed and at thesame time lattice relaxation is performed sufficiently. Therefore,epitaxial growth of a strained Si layer of good quality can be performedon the surface.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a view showing an example of steps for cleaning a multilayersubstrate according to the present invention.

FIG. 2 is a view showing another example of steps for cleaning amultilayer substrate according to the present invention.

FIG. 3 is a view showing steps for bonding a multilayer substrateaccording to the present invention.

FIG. 4 is a view showing an example of steps for producing a bondedwafer according to the present invention.

FIG. 5 is a view showing another example of steps for producing a bondedwafer according to the present invention.

FIG. 6 is a view showing surface energy values when each sample wafer ofExamples 1, 2 and Comparative Examples 1-3 and a base wafer is bonded.

FIG. 7 is a view of showing etching amount of an outermost surface layerof each sample wafer of Example 3 and Comparative Example 4 in the casethat liquid temperature of a SC-1 cleaning liquid is changed.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be explained.However, the present invention is not limited thereto.

As mentioned above, when a strained Si layer is formed, in the case thata method for bonding a surface of a SiGe layer formed on a siliconsubstrate to be a bond wafer to another silicon substrate through anoxide film, in order to remove particles or contaminants on surfaces ofthe bonded plane before bonding, the SiGe layer is subjected to SC-1cleaning and such which is used for cleaning of a general siliconsubstrate. In this case, because Ge etching rate is larger than Sietching rate, surface roughness of the SiGe layer after being cleaned isrougher than that in the case that a silicon surface is subjected toSC-1 cleaning, and the surface roughness becomes larger along with Geconcentration being higher. Because the strain of the strained Si layercan be larger as the Ge concentration is higher, it is desirable thatthe Ge concentration of the SiGe layer is high. However, surfaceroughness after the cleaning becomes further larger. In a bondedsubstrate that is produced by bonding a surface of the surface-roughenedSiGe layer to a silicon substrate to be a base wafer through an oxidefilm, a bonding force of the bonded plane lowers. For example,thereafter, bonding defects are caused at the bonded plane in a step ofthinning a silicon substrate of the bond wafer and such, and lowering ofprocess yield of producing bonded substrates is provoked.

With respect to this, the present inventors have found that if aprotective film is formed on a surface of the SiGe layer and then theprotective film is etched and cleaned with a first cleaning liquid sothat the protective film remains, roughening of the surface can beprevented because the SiGe layer is protected during cleaning. Andadditionally because the protective film is slightly etched and removedwith the first cleaning liquid, the cleaning effect can be high andbonding force of the bonded plane can be prevented from lowering.Moreover, if the multilayer substrate cleaned with the first cleaningliquid is cleaned with a second cleaning liquid which is capable ofetching the protective film and which has a smaller etching rate for theprotective film than the first cleaning liquid so that the protectivefilm is removed and that the SiGe layer is exposed, the SiGe layer canbe etched with the second cleaning liquid having a small etching rate sothat roughening of the surface is not caused. Therefore, roughening of asurface of the SiGe layer to be a bonded plane is prevented and at thesame time the cleaning effect can be drastically high, thereby bondingforce of the bonded plane can be prevented from lowering.

On the other hand, as mentioned above, in the case that an SOI wafer isproduced by using an ion implantation delamination method, it iscontaminated by organic matter or metal impurities adhering to a surfaceof a Si_(1-X)Ge_(X) layer when ions are implanted. Furthermore,roughening of the surface is also caused on the surface of theSi_(1-X)Ge_(X) layer, and in the case that a surface of theSi_(1-X)Ge_(X) layer and a base wafer are closely superposed through anoxide film after the ion implantation, there has been a problem thatbonding defects such as voids or blisters are generated in the bondedplane after the delaminating heat treatment.

However, if a surface of the Si_(1-X)Ge_(X) layer is subjected to SC-1cleaning for removing the organic matter or metal impurities, surfaceroughness of the Si_(1-X)Ge_(X) layer after the cleaning becomes furtherlarger by the above-described reason. There has been a problem that whena surface of the Si_(1-X)Ge_(X) layer or the like after being cleanedand a base wafer are closely superposed through an oxide film, bondingdefects such as voids or blisters are generated in the bonded planeafter the delaminating heat treatment.

Moreover, in the case that the Si_(1-X)Ge_(X) layer is bonded to a basewafer through a Si layer or an insulator layer thereon, latticerelaxation within the condensation SiGe layer when Ge concentration isenhanced in the Si_(1-X)Ge_(X) layer is not performed sufficiently.Therefore, dislocations are generated and the strained Si layer lyingdirectly thereon becomes also bad in crystallinity.

With respect to this, the present inventors have found that if aprotective layer is formed on a surface of a Si_(1-X)Ge_(X) layer(0<X<1), at least one kind of a hydrogen ion and rare gas ions isimplanted through the protective layer thereby to form an ion implantedlayer and then the bond wafer formed with the ion implanted layer iscleaned, roughening of the surface of the Si_(1-X)Ge_(X) layer bycleaning can be prevented with the protective layer and at the same timeorganic matter or metal impurities adhering to the implanted surfacewhen ions are implanted can be removed. Therefore, voids or blisters canbe prevented from being generated at the bonded plane after delaminatingheat treatment. Moreover, if a surface of the protective layer of thebond wafer after being cleaned and a base wafer are closely superposedthrough an insulator oxide film or directly, it becomes easy to causeslipping on the interface between the protective layer and the basewafer, and in a condensation SiGe layer formed by Ge being condensed atthe Si_(1-X)Ge_(X) layer when the surface of the delaminated layer issubsequently subjected to thermal oxidation thereby to form a thermaloxide film, generation of dislocations is suppressed and at the sametime lattice relaxation is performed sufficiently. Thus, the presentinvention has been accomplished.

Hereinafter, embodiments of the present invention will be explained byusing drawings.

FIG. 1 is a view showing an example of steps for cleaning a multilayersubstrate according to the present invention.

First, a multilayer substrate 1 having a SiGe layer 2 as an uppermostsurface layer is prepared as in FIG. 1( a). This multilayer substrate 1is not particularly limited as long as having a SiGe layer as anuppermost surface layer. However, such a substrate that epitaxial growthof a SiGe layer is performed on a surface of the silicon single crystalwafer can be used.

Next, as shown in FIG. 1( b), a protective film 3 is formed on the SiGelayer 2. As the protective film 3, for example, a silicon singlecrystal, a silicon polycrystal, an amorphous silicon, a silicon oxide,and so forth can be used.

And, as shown in FIG. 1( c), the protective film 3 is cleaned with afirst cleaning liquid. The cleaning can be performed, for example, byimmersing the multilayer substrate 1 in the first cleaning liquid. Thefirst cleaning liquid can etch the protective film 3 and, for example,if the protective film 3 consists of silicon, a SC-1 cleaning liquidconsisting of a mixed aqueous solution of NH₄OH and H₂O₂ can be used asthe first cleaning liquid. Because the protective film 3 is slightlyetched by the cleaning, the cleaning effect can be high. In this case,by performing the cleaning so that the protective film 3 remains, a SiGelayer 2 can be protected and roughening of the surface can be prevented.Therefore, bonding force of the bonded plane can be prevented fromlowering.

In this case, it is preferable that the cleaning is performed so that athickness of the remaining protective film 3 is from 1 nm to 100 nm. Ifa thickness of the remaining protective film 3 is 1 nm or more, itdoesn't happen that a part of the SiGe layer 2 is exposed on the surfacethereof and roughening of the surface is partially caused during thecleaning, and the thickness is sufficient for protecting the SiGe layer2 from roughening of the surface by cleaning. Moreover, if the thicknessis 100 nm or less, also in the case that a bonded SOI wafer having astrained Si layer is produced by being bonded to a base wafer having anoxide film on a surface thereof at a later step, a thickness from thestrained Si layer to the oxide film can be sufficiently thin. Thelowering effect of stray capacitance that is an advantage of SOIstructure can be sufficient.

In this case, the thickness of the remaining protective film 3 can beeasily adjusted by adjusting composition or temperature of the firstcleaning liquid or the cleaning time. For example, if the temperature ofthe cleaning liquid is low and the cleaning time is shortened, etchingamount in the cleaning can be small. By adjusting these conditions, thethickness of the remaining protective film 3 can be, for example, from 1nm to 100 nm.

FIG. 2 is a view showing another example of steps for cleaning amultilayer substrate according to the present invention.

FIG. 2( a)-(c) can be performed in the manner similar to the steps asshown in FIG. 1( a)-(c). Then, as in FIG. 2( d), cleaning is performedwith a second cleaning liquid so that a remaining protective film 3′after the cleaning with a first cleaning liquid is removed and that aSiGe layer 2′ is exposed. The cleaning can be performed, for example, byimmersing the multilayer substrate 1′ in the second cleaning liquid. Thesecond cleaning liquid is capable of etching the protective film 3′ andhas a smaller etching rate for the protective film than the firstcleaning liquid and, for example, if the protective film 3′ consists ofsilicon, a SC-1 cleaning liquid consisting of a mixed aqueous solutionof NH₄OH and H₂O₂ adjusted to have a slower etching rate than the firstcleaning liquid may be used as the second cleaning liquid. By performingthe cleaning with the second cleaning liquid so that the protective film3′ is removed and that the SiGe layer 2′ is exposed, the SiGe layer 2′can be etched and cleaned so that roughening of the surface is notcaused. Therefore, roughening of a surface of the SiGe layer 2′ to be abonded plane is prevented and at the same time the cleaning effect canbe very high, therefore bonding force of the bonded plane can becertainly prevented from lowering.

The second cleaning liquid has a smaller etching rate than the firstcleaning liquid. For example, if a temperature of the second cleaningliquid is lower than a temperature of the first cleaning liquid, theetching rate of the second cleaning liquid can be easily made to besmall so that roughening of the surface of the SiGe layer 2′ is notcaused. Of course, the etching rate may be small by adjustingcomposition of the second cleaning liquid.

FIG. 3 is a view showing an example of steps for bonding a multilayersubstrate according to the present invention. First, a multilayersubstrate 1″ cleaned by using the above-mentioned cleaning method isprepared. The uppermost surface layer 4 of the multilayer substrate 1″is the SiGe layer or the protective film remaining slightly thereon andhas a surface in which roughening of the surface is prevented by thecleaning. Moreover, another substrate 5 (a base wafer) to be bonded tothe multilayer substrate 1″ is prepared. The base wafer 5 may be, forexample, a silicon single crystal wafer that an oxide film is formed ona surface thereof, or an insulator wafer such as quartz, siliconcarbide, alumina, or diamond. It is preferable that each of them has asurface that is cleaned in order to prevent bonding force from loweringin a bonded plane thereof to the multilayer substrate 1″ and that has asurface in which roughening of the surface is small.

Next, the multilayer substrate 1″ and a base wafer 5 are bonded. Thebonding can be performed at a room temperature and then binding force isenhanced by heat-treating, for example, at about 200-1200° C. in anitrogen atmosphere. If the multilayer substrate 1″ is bonded to thebase wafer 5 as mentioned above, bonding force can be prevented fromlowering by roughening of the surface or contamination in the bondedplane. Therefore, troubles such as delamination at the bonded plane donot happen in subsequent steps. Thereby, improvement of process yield ofproducing a bonded substrate is achieved.

FIG. 4( a)-(i) is a view showing an example of steps for producing abonded wafer according to the present invention.

First, as in FIG. 4( a), a Si_(1-X)Ge_(X) layer 12 is grown at athickness of about 10-500 nm on a silicon single crystal wafer 11 byvapor growth method. In this case, X in the Si_(1-X)Ge_(X) layer 12, orGe composition, can be constant. For example, if a Si_(1-X)Ge_(X) layeris formed as a gradient composition layer that X=0 in early phase of thegrowth and the X is gradually increased toward a surface, dislocationsgenerated in the Si_(1-X)Ge_(X) layer can be effectively suppressed. Inaddition, for suppressing dislocations, X<0.2 is preferable, and ifX≦0.15, dislocations can be sufficiently suppressed.

The vapor growth can be performed by CVD (Chemical Vapor Deposition)method, MBE (Molecular Beam Epitaxy) method, or the like. In the case ofthe CVD method, for example, a mixed gas of SiH₄ or SiH₂Cl₂ and GeH₄ canbe used as a material gas. H₂ can be used as a carrier gas. Asconditions for the growth, for example, temperature may be 600-1000° C.and pressure 100 Torr or less.

Next, as shown in FIG. 4( b), a protective layer 13 is formed on asurface of the grown Si_(1-X)Ge_(X) layer 12. As the protective layer13, a silicon single crystal layer, an amorphous silicon layer, apolysilicon layer, a silicon oxide film layer, or the like can be used.Any one of these protective layers can be formed by vapor growth method.If the protective layer is a silicon oxide film layer, it can be formedby thermal oxidation and can be used as a BOX (Buried OXide film) layerafter the bonding. Moreover, in the case that the protective layer is asilicon single crystal layer, it can be used as a SiGe layer in which Geis condensed in a later step. It is preferable that a thickness of theprotective layer 13 is an extent that slipping is sufficiently caused atthe bonded plane. For example, if the protective layer 13 is a siliconoxide film layer, a thickness may be 100 nm or less, preferably, 50 nmor less.

And, as shown in FIG. 4( c), at least one kind of a hydrogen ion andrare gas ions is implanted through the protective layer 13 at apredetermined dose amount thereby to form an ion implanted layer 14inside the silicon single crystal wafer 11. In this case, because theion-implanted depth depends on magnitude of the implantation energy, theimplantation energy may be set so as to achieve the predeterminedimplantation depth. In addition, the ion implantation depth in thepresent invention may be to the inside of the silicon single crystalwafer 11 that is a bond wafer, to the interface of the bond wafer andthe Si_(1-X)Ge_(X) layer 12, or to the inside of the Si_(1-X)Ge_(X)layer 12 as mentioned later. In brief, as a delaminated layer afterbonding, at least one part of the Si_(1-X)Ge_(X) layer 12 may betransferred to the base wafer.

Moreover, if ion implantation is performed from a directionperpendicular to a surface of the protective layer 13, channeling can beprevented and in-plane uniformity of the ion implantation depth can beenhanced. Therefore, the ion implanted layer 14 of good quality can beformed. In particular, if the protective layer 13 is a silicon oxidefilm layer, an amorphous silicon layer, a polysilicon layer, or thelike, channeling can be prevented effectively from being caused.

Next, as shown in FIG. 4( d), by cleaning the bond wafer, a surface ofthe protective layer 13 is cleaned, and organic matter and metalimpurities on the surface are removed. The cleaning may be performed inthe same manner as the cleaning of the protective film as shown in FIG.1( c) that is an example of a cleaning step of multilayer substrateaccording to the present invention as described above. The cleaning canbe performed by SC-1 cleaning in the same conditions that are generallyused in cleaning of a Si wafer, and further SC-1 cleaning and SC-2cleaning (cleaning with a mixed aqueous solution of HCl and H₂O₂) can beappropriately combined and performed. Moreover, by combining a sulfuricacid-hydrogen peroxide solution cleaning (cleaning with a mixed aqueoussolution of H₂SO₄ and H₂O₂) or an ozone water cleaning with thesecleanings, removing effect of organic matter can be enhanced. In thecase that the Si_(1-X)Ge_(X) layer 12 is exposed on the surface, ifgeneral cleaning of a silicon substrate as described above is performed,roughening of the surface is caused on the Si_(1-X)Ge_(X) layer 12.However, roughening of the surface is not caused in the presentinvention because the Si_(1-X)Ge_(X) layer 12 is protected by theprotective layer 13. Therefore, generation of bonding defects such asvoids or blisters in subsequent steps can be prevented.

In this case, a base wafer 15 that is separately prepared may besubjected to the similar cleaning. If the base wafer 15 to be preparedis a silicon single crystal wafer or the like, a silicon oxide film 16is formed on a surface thereof. The silicon oxide film 16 to be formedis finally to become a BOX layer when a bonded wafer is completed.Therefore, for obtaining the film of high quality, it is preferable thatthe film is formed by thermal oxidation. Moreover, in the case that asthe base wafer 15, an insulator wafer such as quartz, silicon carbide,alumina, or diamond is used, the silicon oxide film 16 does not alwayshave to be formed on the surface. However, a silicon oxide film can beformed by a CVD method or the like.

Next, as shown in FIG. 4( e), a surface of the protective layer 13 and asurface of the base wafer 15 are superposed at a room temperaturethrough the silicon oxide film 16. If the base wafer 15 is an insulatorwafer such as quartz, the protective layer 13 and the base wafer 15 maybe superposed directly. In this case, because the interface between theprotective layer 13 and the silicon oxide film 16 or the insulator basewafer 15 becomes a bonded plane, slipping is easily caused at the bondedplane, and lattice relaxation of a condensation SiGe layer 19 in whichGe concentration is enhanced by formation of a thermal oxide film 18 atthe later step is easily performed. Thereby generation of dislocationsin the condensation SiGe layer 19 can be suppressed.

Next, as shown in FIG. 4( f), heat treatment (delaminating heattreatment), for example, at 500° C. or more is carried out and therebydelamination is performed so that the ion implanted layer 14 becomes acleavage plane. Thereby, a part of a silicon single crystal wafer 17,the Si_(1-X)Ge_(X) layer 12, and the protective layer 13 are transferredto the base wafer side as a delaminated layer.

In addition, as pretreatment of the step of closely superposing asurface of the protective layer 13 and a surface of the base wafer 15 asshown in FIG. 4( e), if the surfaces provided for closely superposingthe both wafers are subjected to plasma treatment and thereby theadhesion strength is enhanced, it is possible that delamination can bemechanically performed at the ion implanted layer 14 without performingthe delaminating heat treatment after the closely superposing.

Next, as shown in FIG. 4( g), the surface of the delaminated layertransferred to the base wafer side is subjected to thermal oxidation andthereby a thermal oxide film 18 is formed. Thermal oxidation in thiscase is performed against the silicon layer 17 and a part of aSi_(1-X)Ge_(X) layer 12 of the delaminated layer. In this case, if apart of the Si_(1-X)Ge_(X) layer 12 of the delaminated layer issubjected to thermal oxidation, Ge is hardly taken in the oxide film.Therefore, Ge existing in the thermally oxidized part transfers to apart that is not thermally oxidized, and a condensation SiGe layer 19 inwhich Ge is condensed is formed. Moreover, in the case that theprotective layer 13 is a silicon single crystal layer, the protectivelayer 13 can be also used as a part of the SiGe layer 19 in which Ge iscondensed. As described above, because Ge concentration in thecondensation SiGe layer 19 can be enhanced by oxidizing theSi_(1-X)Ge_(X) layer 12, stronger strain (compression strain) isgenerated in the condensation SiGe layer 19. However, because a bondedinterface in which chemical bond is not perfect exists near thecondensation SiGe layer 19, slipping is generated so as to relax thestrain of the condensation SiGe layer 19 in the interface, and latticerelaxation is accomplished with suppressing generation of dislocationsin the condensation SiGe layer 19.

In this case, because damage caused by the ion implantation remains inthe surface of the silicon layer 17, OSF are easily generated bysubjecting a surface of the delaminated layer directly to thermaloxidation at a higher temperature than 1000° C. Therefore, it ispreferable that until the damaged layer is taken in by a thermal oxidefilm in the thermal oxidation of the silicon layer 17, the thermaloxidation is performed at a temperature of 1000° C. or less, preferably950° C. or less. Alternatively, the thermal oxidation is performed afterthe delaminated surface is slightly polished (touch-polished).

On the other hand, in the case that the Si_(1-X)Ge_(X) layer 12 issubjected to thermal oxidation, Ge is condensed in the condensation SiGelayer 19 by the thermal oxidation because Ge is hardly taken into theoxide film as described above. If the thermal oxidation temperature isless than 900° C., Ge precipitation becomes easily caused at theinterface of the thermal oxide film 18 and the condensation SiGe layer19. Therefore, it is desirable that the oxidation temperature is 900° C.or more, preferably 1000° C. or more. Moreover, it is possible that heattreatment under a non-oxidizing atmosphere such as Ar, H₂, and N₂ isadded after the oxidation and thereby Ge is diffused so that Geconcentration in the depth direction becomes uniform.

That is, in the case that delamination is performed by forming an ionimplanted layer inside the silicon single crystal wafer 11 and a surfaceof the delaminated layer is a silicon layer 17, the suitable steps isthat thermal oxidation is performed at a temperature of 1000° C. orless, preferably 950° C. or less until the whole silicon layer 17 turnsinto thermal oxide film, and then when the Si_(1-X)Ge_(X) layer 12existing at the lower part of the silicon layer 17 is subjected tothermal oxidation, oxidation is performed at a temperature of 900° C. ormore, preferably 1000° C. or more.

Next, as shown in FIG. 4( h), the formed thermal oxide film 18 isremoved and the lattice-relaxed condensation SiGe layer 19 is exposed.For removing the thermal oxide film, a HF aqueous solution can be used.

Last, as shown in FIG. 4( i), epitaxial growth of the silicon singlecrystal layer 20 is performed by a vapor growth method on a surface ofthe exposed condensation SiGe layer 19. The epitaxial growth can beperformed by CVD method, MBE method, or the like. In the case of CVDmethod, for example, SiH₄ or SiH₂Cl₂ can be used as a material gas. Asconditions of the growth, temperature may be 600-1000° C. and pressure100 Torr or less. The formed silicon single crystal layer 20 becomes astrained Si layer having tensile strain inside by difference in latticeconstant from the condensation SiGe layer 19 that is the lower layerthereof. Because it is formed on the condensation SiGe layer 19 of goodquality having few dislocations, it becomes a strained Si layer of goodquality. It is preferable that a thickness of the silicon single crystallayer 20 to be epitaxially grown is about 10-50 nm for ensuringeffective strain, workability and quality in device fabrication.

Next, FIG. 5( a)-(i) is a view showing another example of steps forproducing a bonded wafer according to the present invention. Formationof a Si_(1-X)Ge_(X) layer 12′ on a surface of the silicon single crystalwafer 11′ and formation of a protective layer 13′ in FIGS. 5( a) and (b)can be performed with steps similar to those in FIGS. 4( a) and (b).

Next, as shown in FIG. 5( c), at least one kind of a hydrogen ion andrare gas ions is implanted through the protective layer 13′ at apredetermined dose amount thereby to form an ion implanted layer 14′inside the Si_(1-X)Ge_(X) layer 12′. In this case, the ion implantedlayer 14′ may be formed in an interface between the Si_(1-X)Ge_(X) layer12′ and the silicon single crystal wafer 11′. Because anion-implantation depth depends on magnitude of the implantation energy,the implantation energy may be set so as to achieve a desiredimplantation depth.

Next, as shown in FIGS. 5( d) and (e), the surface of the protectivelayer 13′ is cleaned to remove organic matter or metal impurities on thesurface. The cleaning may be in a manner similar to the above-mentionedcleaning of a protective film as shown in FIG. 1( c). A base wafer 15′that is separately prepared is subjected to the similar cleaning. Then,the surface of the protective layer 13′ and a surface of the base wafer15′ are closely superposed through a silicon oxide film 16′ or directlyat a room temperature. These steps can be performed according to thesteps similar to those in FIGS. 4. (d) and (e).

Next, as shown in FIG. 5( f), heat treatment (delaminating heattreatment), for example, at 500° C. or more is carried out and therebydelamination is performed so that the ion implanted layer 14′ becomes acleavage plane. Thereby, a part or the whole of the Si_(1-X)Ge_(X) layer17′, and the protective layer 13′ are transferred to the base waferside. In addition, also in this case, as a pretreatment of the step ofclosely superposing a surface of the protective layer 13′ and a surfaceof the base wafer 15′, the surfaces provided for closely superposing theboth wafers is plasma-treated to enhance the adhesion strength, therebydelamination may be mechanically performed at the ion implanted layer14′ without performing the delaminating heat treatment.

Next, as shown in FIG. 5( g), a surface of the Si_(1-X)Ge_(X) layer 17′transferred to the base wafer side is subjected to thermal oxidation andthereby a thermal oxide film 18′ is formed. In this case, by theformation of the thermal oxide film 18′, the condensation SiGe layer 19′in which Ge is condensed. Moreover, in the case that the protectivelayer 13′ is a silicon single crystal layer, the protective layer 13′can be also used as a part of the SiGe layer 19′ in which Ge iscondensed. Stronger strain (compression strain) is generated in thecondensation SiGe layer 19′. However, because a bonded interface existsnear the condensation SiGe layer 19′, and slipping is generated so as torelax the strain of the condensation SiGe layer 19′ in the interface,thereby lattice relaxation is accomplished with suppressing generationof dislocations in the condensation SiGe layer 19′.

Also, in this case, Ge is condensed in the condensation SiGe layer 19′by the thermal oxidation because Ge is hardly taken into the oxide filmas described above. If the thermal oxidation temperature is less than900° C., Ge precipitation becomes easily caused at the interface of thethermal oxide film 18′ and the condensation SiGe layer 19′. Therefore,it is desirable that the oxidation temperature is 900° C. or more,preferably 1000° C. or more.

Moreover, it is desirable that after touch-polishing a damaged layer ofa surface of the transferred Si_(1-X)Ge_(X) layer 17′, the transferredSi_(1-X)Ge_(X) layer 17′ is subjected to thermal oxidation at anoxidation temperature of 900° C. or more, preferably 1000° C. or more,and thereby Ge condensation is performed. In this case, there is not aSi layer on a surface of the delaminated layer, and a problem that OSFare generated is not caused. Therefore, heat treatment may beimmediately performed at a temperature of 1000° C. or more.

Next, as shown in FIG. 5( h), the formed thermal oxide film 18′ isremoved and the lattice-relaxed condensation SiGe layer 19′ is exposed.For removing the thermal oxide film 18′, a HF aqueous solution can beused.

Last, as shown in FIG. 5( i), epitaxial growth of a silicon singlecrystal layer 20′ is performed by vapor growth method on a surface ofthe exposed condensation SiGe layer 19′. The silicon single crystallayer 20′ formed as described above becomes a strained Si layer havingtensile strain inside by difference in lattice constant from thecondensation SiGe layer 19′ that is the lower layer thereof. Because itis formed on the condensation SiGe layer 19′ of good quality having fewdislocations, it becomes a strained Si layer of good quality. It ispreferable that a thickness of the silicon single crystal layer 20′ tobe epitaxially grown is about 10-50 nm for ensuring effective strain,workability and quality in device fabrication.

Hereinafter, the present invention is explained in detail according toExamples and Comparative Examples. However, the present invention is notlimited to these.

Examples 1 and 2 Comparative Examples 1-3

A total of 4 kinds of sample wafers were prepared: on a surface (amirror-polished surface) of a silicon single crystal wafer having adiameter of 200 mm, a SiGe layer with a Ge concentration of 5% or 15% isdeposited only by 50 nm by epitaxial method to be an uppermost surfacelayer as shown in Table 1 as described below (Comparative Examples 1 and2), and as a protective film, a protective silicon layer is furtherdeposited thereon only by 20 nm by epitaxial method to be an uppermostsurface layer (Examples 1 and 2). Moreover, for a reference, a generalmirror-polished silicon single crystal wafer without the above-describedepitaxial layer being formed (Comparative Example 3) was prepared.

The surface of each uppermost surface layer of the five kinds of thesample wafers and silicon single crystal base wafers that wereseparately prepared (having a thermal oxide film with a thickness of 400nm) were subjected to SC-1 cleaning in conditions as described below.Then bonding was performed at a room temperature and heat treatment wasperformed at 350° C. for 2 hours (in a nitrogen atmosphere). Then,surface energy in each bonded interface that is proportional to bondingforce was evaluated by a razor-blade insertion method.

TABLE 1 Protective Wafer SiGe Layer Si Layer Structure (50 nm) (20 nm)(from Surface) Example 1 Existence: Existence Protective Si/ Ge 5%SiGe/Si Wafer Example 2 Existence: Existence Protective Si/ Ge 15%SiGe/Si Wafer Comparative Existence: None SiGe/Si Wafer Example 1 Ge 5%Comparative Existence: None SiGe/Si Wafer Example 2 Ge 15% ComparativeNone None Si Wafer Example 3 <SC-1 cleaning conditions> Composition 29wt % NH₄OH:30 wt % H₂O₂:H₂O = 1:1:5 (capacity ratio) Liquid Temperature80° C. Cleaning Time 3 minutes

In Table 2 and FIG. 6, results of measuring an RMS (Root Mean Square)value of surface roughness of 1-μm-square and 10-μm-square in anuppermost surface layer near a central portion of each sample waferafter the SC-1 cleaning by AFM (Atomic Force Microscope), and results ofmeasuring the surface energy by a razor-blade insertion method in thecase of bonding each said sample wafer and a base wafer are shown.Moreover, a film-thickness of an uppermost surface layer of each saidsample wafer before and after the SC-1 cleaning was measured, andresults of calculating etching amount of the uppermost surface layer bythe cleaning are shown in Table 3.

TABLE 2 Surface Roughness after the Surface Cleaning (RMS value) (nm)Energy 1-μm-square 10-μm-square (J/m²) Example 1 0.129 0.087 1.949Example 2 0.144 0.087 1.9 Comparative 0.157 0.103 1.777 Example 1Comparative 0.209 0.121 1.578 Example 2 Comparative — — 1.911 Example 3

TABLE 3 Film-Thickness of Surface Layer (nm) Etching Before the Afterthe Amount Cleaning Cleaning (nm) Example 1 18.93 17.39 1.54 Example 220.53 18.72 1.81 Comparative 46.43 43.99 2.44 Example 1 Comparative50.11 45.23 4.88 Example 2

As shown in FIG. 2, with regard to the surface roughness after thecleaning, values of Examples 1 and 2 are smaller than that ofComparative Examples 1 and 2. For example, compared at the Geconcentration of 15%, the RMS value of 1-μm-square in Example 2 was0.144 nm, and that in Comparative Example 2 was 0.209 nm, which waslargely different from the former. From this result, it was found thatroughening of the surface at the uppermost surface layer by the cleaningwas prevented in Examples 1 and 2. Moreover, with regard to the surfaceenergy, as shown in Table 2 and FIG. 6, values in Examples 1 and 2 werelarger than those of Comparative Examples 1 and 2. For example, while itwas 1.578 J/m² in Comparative Example 2, it was 1.9 J/m² in Example 2,which was an equivalent good value to 1.911 J/m² in the case of havingno SiGe layer in Comparative Example 3. From these results, it was foundthat bonding force is prevented from lowering at a bonded plane by thecleaning.

Moreover, as shown in Table 3, even in the same cleaning conditions,etching amount of Examples 1 and 2 was smaller than that of ComparativeExamples 1 and 2, and it was confirmed that etching rate is higher insamples having larger surface roughness.

Example 3 Comparative Example 4

By using a sample wafer (Example 3) produced in the same conditions asin Example 2 as shown in Table 1 and a sample wafer (Comparative Example4) produced in the same conditions as in Comparative Example 2, SC-1cleaning was performed with changing only a liquid temperature in theabove-mentioned SC-1 cleaning conditions to 25° C., 50° C., and 80° C.,and thereby etching amount of the uppermost surface layer of each saidsample wafer was compared. The results were shown in Table 4 and FIG. 7.From Table 4 and FIG. 7, etching amount in Example 3 was smaller thanthat in Comparative Example 4 at each liquid temperature. In addition,as the liquid temperature was higher, the difference between the two waslarger. Moreover, by setting the liquid temperature of the SC-1 cleaningliquid to be lower, the etching amount can be smaller and it wasconfirmed that the etching amount can be adjusted by adjusting theliquid temperature.

TABLE 4 Etching Amount (nm) 25° C. 50° C. 80° C. Example 3 0.06 0.411.81 Comparative Example 4 0.09 1.01 4.88

Next, the sample wafer of Example 2 (the protective Si layer: 20 nm) wassubjected to SC-1 cleaning at a liquid temperature of 80° C. for 10minutes to remove a Si layer of the surface layer part by about 18 nm.Then, SC-1 cleaning was sequentially performed for 6 minutes at a liquidtemperature of 50° C., lower temperature than before. And thereby, aSiGe layer was exposed. Surface roughness of the exposed SiGe layer wasmeasured by AFM in the same manner as in Examples 1 and 2. Moreover,surface energy was measured by the same method as in Examples 1 and 2.As a result, surface roughness was 0.15 nm at 1-μm-square, 0.09 nm at10-μm-square, respectively. Moreover, surface energy was 1.88 J/m² andthe good result similar to the Example 2 was obtained.

Example 4

A Si_(0.97)Ge_(0.03) layer (X=0.03) was grown by about 150 nm on asurface of a silicon single crystal wafer with a diameter of 200 mm byCVD method. And a protective layer of the silicon single crystal wasformed by 50 nm on the surface of the Si_(0.97)Ge_(0.03) layer by CVDmethod. Hydrogen ions (H⁺) were ion-implanted through the protectivelayer of the silicon single crystal in conditions that the implantationenergy was 40 keV and the dose amount was 5×10¹⁶ atoms/cm², thereby anion-implanted layer was formed on the surface layer part of the siliconsingle crystal wafer. After the hydrogen ion implantation, the surfaceof the protective layer of the silicon single crystal was subjected tocleaning with a sulfuric acid-hydrogen peroxide solution at 120° C. for5 minutes, and SC-1 cleaning was sequentially performed at 80° C. for 3minutes. Then, the wafer was closely superposed at a room temperature ona silicon single crystal base wafer having a thermal oxide film with 400nm cleaned in the same conditions, and delaminating heat treatment wasperformed under an argon atmosphere at 500° C. for 30 minutes to carryout delamination at the ion implanted layer, and thereby the protectivelayer of the silicon single crystal, the Si_(0.97)Ge_(0.03) layer, and apart of the silicon layer were transferred to the base wafer side. Next,thermal oxidation was performed at 950° C. and the silicon layer wasthermally oxidized, and then the temperature was sequentially elevatedto 1100° C. and a part of the Si_(0.97)Ge_(0.03) layer was thermallyoxidized. Thereby, a condensation SiGe layer in which Ge concentrationwas 20% or more was formed. Then, the oxide film was removed with 5% HFaqueous solution, the condensation SiGe layer was exposed, and theepitaxial growth of a silicon layer was performed on the surface by athickness of 50 nm by CVD method.

20 bonded wafers produced as described above were prepared, and theirsurfaces were observed with eyes and the generation number of voids andblisters was counted. As a result, the generation number of voids andblisters per wafer was about 0.5.

Example 5

A Si_(0.97)Ge_(0.03) layer (X=0.03) was grown by about 150 nm on asurface of a silicon single crystal wafer with a diameter of 200 mm byCVD method. An amorphous silicon protective layer was formed by 50 nm onthe surface of Si_(0.97)Ge_(0.03) layer. Hydrogen ions (H⁺) wereion-implanted through the amorphous silicon protective layer inconditions that the implantation energy was 40 keV and the dose amountwas 5×10¹⁶ atoms/cm², thereby an ion-implanted layer was formed on thesurface layer part of the silicon single crystal wafer. After thehydrogen ion implantation, the surface of the amorphous siliconprotective layer was subjected to SC-1 cleaning at 80° C. for 3 minutes,SC-2 cleaning at 80° C. for 3 minutes, and SC-1 cleaning at 80° C. for 3minutes in order. Then, it was closely superposed at a room temperatureon a silicon single crystal base wafer having a thermal oxide film with400 nm cleaned in the same conditions, and delaminating heat treatmentwas performed under an argon atmosphere at 500° C. for 30 minutes tocarry out delamination at the ion implanted layer, thereby the amorphoussilicon protective layer, the Si_(0.97)Ge_(0.03) layer, and a part ofthe silicon layer were transferred to the base wafer side. Next, thermaloxidation was performed at 950° C. and the silicon layer was thermallyoxidized. And then the temperature was sequentially elevated to 1100° C.and a part of the Si_(0.97)Ge_(0.03) layer was thermally oxidized.Thereby, a condensation SiGe layer in which Ge concentration was 20% ormore was formed. Then, the oxide film was removed with 5% HF aqueoussolution, the condensation SiGe layer was exposed, and the epitaxialgrowth of a silicon layer was performed on the surface of thecondensation SiGe layer by a thickness of 50 nm by CVD method.

20 bonded wafers produced as described above were prepared, and theirsurfaces were observed with eyes and the generation number of voids andblisters was counted. As a result, the generation number of voids andblisters per wafer was about 0.8.

Example 6

A Si_(0.97)Ge_(0.03) layer (X=0.03) was grown by about 150 nm on asurface of the silicon single crystal wafer with a diameter of 200 mm byCVD method. And, a protective layer of the silicon single crystal wasformed by 50 nm on the surface of the Si_(0.97)Ge_(0.03) layer by CVDmethod. Hydrogen ions (H⁺) were ion-implanted through the protectivelayer of the silicon single crystal in conditions that the implantationenergy was 15 keV and the dose amount was 5×10¹⁶ atoms/cm², thereby anion-implanted layer was formed inside the Si_(0.97)Ge_(0.03) layer.After the hydrogen ion implantation, the surface of the protective layerof the silicon single crystal was subjected to cleaning with a sulfuricacid-hydrogen peroxide solution at 120° C. for 5 minutes, and SC-1cleaning was sequentially performed at 80° C. for 3 minutes. Then, itwas closely superposed at a room temperature on a silicon single crystalbase wafer having a thermal oxide film with 400 nm cleaned in the sameconditions, and delaminating heat treatment was performed under an argonatmosphere at 500° C. for 30 minutes to carry out delamination at theion implanted layer, thereby the protective layer of the silicon singlecrystal, and a part of the Si_(0.97)Ge_(0.03) layer were transferred tothe base wafer side. Next, thermal oxidation was performed at 1100° C.and a part of the Si_(0.97)Ge_(0.03) layer was thermally oxidized. Then,the oxide film was removed with 5% HF aqueous solution to expose thecondensation SiGe layer, and the epitaxial growth of a silicon layer wasperformed on the surface of the condensation SiGe layer by a thicknessof 50 nm by CVD method. 20 bonded wafers produced as described abovewere prepared, and their surfaces were observed with eyes and thegeneration number of voids and blisters was counted. As a result, thegeneration number of voids and blisters per wafer was about 0.5.

Comparative Example 5

A Si_(0.97)Ge_(0.03) layer was grown by about 150 nm on a surface of asilicon single crystal wafer with a diameter of 200 mm by CVD method.Hydrogen ions (H⁺) were ion-implanted through the Si_(0.97)Ge_(0.03)layer in conditions that the implantation energy was 40 keV and the doseamount was 5×10¹⁶ atoms/cm² thereby an ion-implanted layer was formed onthe surface of the silicon single crystal wafer. After the hydrogen ionimplantation, the surface of the Si_(0.97)Ge_(0.03) layer was subjectedto cleaning with a sulfuric acid-hydrogen peroxide solution at 120° C.for 5 minutes, and sequentially SC-1 cleaning at 80° C. for 3 minutes.Then, it was closely superposed at a room temperature on a siliconsingle crystal base wafer having a thermal oxide film with 400 nmcleaned in the same conditions, and delaminating heat treatment wasperformed under an argon atmosphere at 500° C. for 30 minutes to carryout delamination at the ion implanted layer, thereby theSi_(0.97)Ge_(0.03) layer, and a part of the silicon layer weretransferred to the base wafer side. Then, by hydrogen annealing at 1200°C., bonding force was enhanced and at the same time the surface wasflattened.

20 bonded wafers produced as described above were prepared, and theirsurfaces were observed with eyes and the generation number of voids andblisters was counted. As a result, the generation number of voids andblisters per wafer was about 8.

That is, the bonded wafers produced by according to the presentinvention has significantly few generation number of voids and blisters.Therefore, the effect of the present invention was confirmed.

The present invention is not limited to the embodiments described above.The above-described embodiments are mere examples, and those having thesubstantially same constitution as that described in the appended claimsand providing the similar working effects are included in the scope ofthe present invention.

1. A method for cleaning a multilayer substrate at least having asilicon single crystal wafer with a SiGe layer epitaxially grown on asurface of the silicon single crystal wafer, wherein the SiGe layer isan outermost surface layer of the substrate, the method comprising:forming a protective film on a surface of the SiGe layer, and thencleaning the multilayer substrate with a first cleaning liquid capableof etching the protective film so that the protective film remains,wherein the protective film prevents roughening of the surface of theSiGe layer while the cleaning is performed, wherein the cleaning isperformed so that a thickness of the remaining protective film is from 1nm to 100 nm.
 2. The method for cleaning a multilayer substrateaccording to claim 1, wherein a composition or a temperature of thefirst cleaning liquid or a cleaning time is adjusted and thereby thethickness of the remaining protective film is adjusted.
 3. The methodfor cleaning a multilayer substrate according to claim 1, wherein themultilayer substrate cleaned with the first cleaning liquid is cleanedwith a second cleaning liquid which is capable of etching the protectivefilm and which has a smaller etching rate for the protective film thanthe first cleaning liquid so that the protective film is removed andthat the SiGe layer is exposed.
 4. A method for bonding substrates,wherein a surface of the SiGe layer or the protective film which is anoutermost surface layer of the multilayer substrate cleaned by using themethod for cleaning a multilayer substrate according to claim 1 and asurface of another substrate are bonded directly or through an insulatorfilm.
 5. The method for cleaning a multilayer substrate according toclaim 1, wherein the protective film consisting of Si is used, and thefirst cleaning liquid consisting of a mixed aqueous solution of NH₄OHand H₂O₂ is used.
 6. The method for cleaning a multilayer substrateaccording to claim 5, wherein the multilayer substrate cleaned with thefirst cleaning liquid is cleaned with a second cleaning liquid which iscapable of etching the protective film and which has a smaller etchingrate for the protective film than the first cleaning liquid so that theprotective film is removed and that the SiGe layer is exposed.
 7. Themethod for cleaning a multilayer substrate according to claim 6, whereinthe second cleaning liquid consisting of a mixed aqueous solution ofNH₄OH and H₂O₂ is used.
 8. The method for cleaning a multilayersubstrate according to claim 7, wherein a temperature of the secondcleaning liquid is lower than a temperature of the first cleaningliquid.
 9. A method for cleaning a multilayer substrate at least havinga silicon single crystal wafer with a SiGe layer epitaxially grown on asurface of the silicon single crystal wafer, wherein the SiGe layer isan outermost surface layer of the substrate, the method comprising:forming a protective film consisting of Si on a surface of the SiGelayer, cleaning the multilayer substrate with a first cleaning liquidconsisting of a mixed aqueous solution of NH₄OH and H₂O₂, which iscapable of etching the protective film so that the protective filmremains, wherein the protective film prevents roughening of the surfaceof the SiGe layer while the cleaning is performed, and then cleaning themultilayer substrate cleaned with the first cleaning liquid with asecond cleaning liquid consisting of a mixed aqueous solution of NH₄OHand H₂O₂, which is capable of etching the protective film and which hasa smaller etching rate for the protective film than the first cleaningliquid so that the protective film is removed and that the SiGe layer isexposed.
 10. The method for cleaning a multilayer substrate according toclaim 9, wherein a composition or a temperature of the first cleaningliquid or a cleaning time is adjusted and thereby a thickness of theremaining protective film is adjusted.
 11. The method for cleaning amultilayer substrate according to claim 9, wherein the cleaning isperformed so that a thickness of the remaining protective film is from 1nm to 100 nm.
 12. The method for cleaning a multilayer substrateaccording to claim 9, wherein a temperature of the second cleaningliquid is lower than a temperature of the first cleaning liquid.
 13. Amethod for bonding substrates, wherein a surface of the SiGe layer orthe protective film which is an outermost surface layer of themultilayer substrate cleaned by using the method for cleaning amultilayer substrate according to claim 9 and a surface of anothersubstrate are bonded directly or through an insulator film.